1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine.
2. Description of Related Art
An internal combustion engine is known, which includes a plurality of combustion chambers and a plurality of fuel injection valves that supply fuel to the respective combustion chambers. In the internal combustion engine, the amount of the fuel supplied to each combustion chamber from the corresponding fuel injection valve is individually controlled. In an internal combustion engine described in Japanese Patent Application Publication No. 2008-297933 (JP-2008-297933 A), in order to suppress a fluctuation in output torque (i.e., a fluctuation in torque output from the internal combustion engine) caused when an ignition timing (i.e., a timing when the fuel in the combustion chamber is ignited by an ignition plug) is retarded for increasing the temperature of a catalyst, the amount of the fuel supplied to each combustion chamber from the corresponding fuel injection valve is individually controlled.
As described above, in the internal combustion engine in which the amount of the fuel supplied to each combustion chamber from the corresponding fuel injection valve is individually controlled, more extensively in the internal combustion engine in which the air-fuel ratio of an air-fuel mixture generated in each combustion chamber is individually controlled, there are cases where, based on the air-fuel ratio of an exhaust gas discharged from one specific combustion chamber (hereinafter this combustion chamber is referred to as “a specific combustion chamber”), the air-fuel ratio of the air-fuel mixture generated in each of the rest of the combustion chambers (i.e., each of the cylinders except the specific combustion chamber; and hereinafter, each of the rest of the combustion chambers is referred to as “a non-specific combustion chamber”) is controlled such that the average air-fuel ratio of the internal combustion engine matches a target air-fuel ratio (hereinafter this control is referred to as “individual air-fuel ratio control”). When the individual air-fuel ratio control is executed, there are cases where the average air-fuel ratio of the internal combustion engine matches the target air-fuel ratio in a state where the air-fuel ratio of the air-fuel mixture generated in the specific combustion chamber (hereinafter this air-fuel ratio is referred to as “a specific air-fuel ratio”) is controlled to be leaner than the target air-fuel ratio and the air-fuel ratio of the air-fuel mixture generated in the non-specific combustion chamber (hereinafter this air-fuel ratio is referred to as “a non-specific air-fuel ratio”) is controlled to be richer than the target air-fuel ratio. In addition, conversely, when the individual air-fuel ratio control is executed, there are cases where the average air-fuel ratio of the internal combustion engine matches the target air-fuel ratio in a state where the specific air-fuel ratio is controlled to be richer than the target air-fuel ratio and the non-specific air-fuel ratio is controlled to be leaner than the target air-fuel ratio.
Further, in a case where the specific air-fuel ratio is intentionally controlled to be leaner than the target air-fuel ratio for a particular purpose when the individual air-fuel ratio control is executed, the average air-fuel ratio of the internal combustion engine matches the target air-fuel ratio in a state where the specific air-fuel ratio is controlled to be leaner than the target air-fuel ratio and the non-specific air-fuel ratio is controlled to be richer than the target air-fuel ratio. Moreover, conversely, in a case where the specific air-fuel ratio is intentionally controlled to be richer than the target air-fuel ratio for a particular purpose when the individual air-fuel ratio control is executed, the average air-fuel ratio of the internal combustion engine matches the target air-fuel ratio in a state where the specific air-fuel ratio is controlled to be richer than the target air-fuel ratio and the non-specific air-fuel ratio is controlled to be leaner than the target air-fuel ratio.
In either case, when the individual air-fuel ratio control is executed, there are cases where the specific air-fuel ratio becomes a relatively lean air-fuel ratio or the non-specific air-fuel ratio becomes a relatively lean air-fuel ratio.
On the other hand, in a situation where the temperature of a catalyst needs to be increased, a misfire is likely to occur in the combustion chamber. Consequently, in a case where the individual air-fuel ratio control is executed when the temperature of the catalyst is increased and the specific or non-specific air-fuel ratio is controlled to a lean air-fuel ratio, a misfire is more likely to occur in the specific or non-specific combustion chamber. In a case where the misfire occurs in the specific or non-specific combustion chamber, exhaust emission characteristics are deteriorated.